rand 
语法: 
   #include <stdlib.h>  int rand( void ); 
功能: 函数返回一个在零到RAND_MAX 之间的伪随机整数。例如:    
srand( time(NULL) );   
for( i = 0; i < 10; i++ )      
    printf( "Random number #%d: %d\n", i, rand() ); 
srand 
语法: 
   #include <stdlib.h>  void srand( unsigned seed ); 
功能: 设置rand()随机序列种子。对于给定的种子seed, rand()会反复产生特定的随机序列。     
srand( time(NULL) );    
for( i = 0; i < 10; i++ )      
    printf( "Random number #%d: %d\n", i, rand() );